Top three bottom three hmmm. alright. Lets do it, I'll be including a 'special mentions' catorgary too.
Top 3. (in no particular order)
Mordin Solus: I love this guy, he can sing, he can shoot he can do SCIENCE! But aside from that he was a surprise. When I saw the character design. I instantly thought, 'cold logical'. IE: That he'd have a hollow voice be a cold uncaring scientist. Instead...we got a face talking, over thinking, emphatic, funny to be around guy. I mean he just caught me by surprise. His story is one of redemption. He doesn't get emo about it though he just deals with it. For me his death was a loss but at the same time the culmination of his character arc. "had to be me, someone else could have gotten wrong." I came to tears when he started singing.
Garrus Vakarian (also known as Space Batman): What more can I say, this guy is awesome. If your a male shep he is your bro. Your brother in arms. You and him have been threw hell walked threw fire and came out smoking. He's cocky (with good reason). He has a strong sense of justice, and (if you did his loyalty mission that way) capable of forgiving even those who wronged him. And if your Femshep romanced him your in for probably one of the best Female Romance options. This is a guy who stays cool as a Brute charges his position but start flirting with him and his cool breaks down. I loved the lines and the scenes.
Legion: Yeah you probably saw this one coming. He was my fav companion in ME2. If you want proof the Geth have a soul and were wronged look no further. Legion sought you out, welded a piece of your armor to it's body. All because you 'opposed the Old Machines'. In your journies and conversations with him you learn, and later confirm, that the Geth/Quarien war was started by the Quareions and not the Geth. The Geth as a whole are infinitely interesting to me. Even with my feelings about the ending, I want to know if now that they have true sentience if a culture with the Quaren (no I can't spell that word worth shit.) will be created. I want to see what they become after this.
Bottom Three.
(jacob isn't on list cause he just sucks)
Miranda: "lalala look at me, I was created to be perfect, lalalala tits, lalalala perfect ass. lalala I have daddy issues." Just like every fuck body else on the team. Sorry, team'sssss.
Ashly: A lot of people say she isn't a raciest. Well if it looks like a duck, and quacks like a duck...it's a raciest. (Look up 'da furher's face for some wtfness and laughs.) Still about five metric fuck tons more interesting then Jacob though...but that's not a big contest.
James/Kaiden: While Kaiden had an interesting background. His anger, resentment, and lack of trust just makes him come across as a giant twat. And James.....While I don't hate him as much as I did the other's on the Bottom three he's just bland. Not Jacob levels of bland. But bland.
Special Mentions: Grunt, Wrex, Thane, Samara, Javik.
Favorite love interests. (besides Liara.)
Tali: TALIIIIIIIIII!!!

Oh comeon who didn't like her. Her romance is heartwarming especially since it could kill her to get her jiggy on. I haven't seen her ME3 romance bits so just from the ME2 I loved it.
Jack: Ahhh jack. A lot of great dialogue is locked unless you paragon romance her. For those that haven't heard it Youtube it cause I'm not gonna spoil it here. Her romance is different from the others. Jack, when it comes to love and being loved. Is broken, and the romance is about putting that broken person back together. This is why she cries in your love scene in ME2 she's letting her guard down and just letting go of her anger. When that was all she had for so long.